The extracellular endo-β-1,4-xylanase with multidomain from the extreme thermophile Caldicellulosiruptor lactoaceticus is specific for insoluble xylan degradation

Functional studies on tandem carbohydrate-binding modules of a multimodular enzyme possessing two catalytic domains

Although functional studies on carbohydrate-binding module (CBM) have been carried out extensively, the role of tandem CBMs in the enzyme containing multiple catalytic domains (CDs) is unclear. Here, we identified a multidomain enzyme (Lc25986) with a novel modular structure from lignocellulolytic bacterial consortium. It consists of a mannanase domain, two CBM65 domains (LcCBM65-1/LcCBM65-2), and an esterase domain. To investigate CBM function and domain interactions, full-length Lc25986 and its variants were constructed and used for enzymatic activity, binding, and bioinformatic analyses. The results showed that LcCBM65-1 and LcCBM65-2 both bind mannan and xyloglucan but not cellulose or β-1,3-1,4-glucan, which differs from the ligand specificity of reported CBM65s. Compared to LcCBM65-2, LcCBM65-1 showed a stronger ligand affinity and a preference for acetylation sites. Both CBM65s stimulated the enzymatic activities of their respective neighboring CDs against acetylated mannan, but did not contribute to the activities of the distal CDs. The time course of mannan hydrolysis indicated that the full-length Lc25986 was more effective in the complete degradation of mixed acetyl/non-acetyl substrates than the mixture of single-CD mutants. When acting on complex substrates, LcCBM65-1 not only improved the enzymatic activity of the mannanase domain, but also directed the esterase domain to the acetylated polysaccharides. LcCBM65-2 adopted a low affinity to reduce interference with the catalysis of the mannanase domain. These results demonstrate the importance of CBMs for the synergism between the two CDs of a multidomain enzyme and suggest that they contribute to the adequate degradation of complex substrates such as plant cell walls.

IMPORTANCE

Lignocellulolytic enzymes, particularly those of bacterial origin, often harbor multiple carbohydrate-binding modules (CBMs). However, the function of CBM multivalency remains poorly understood. This is especially true for enzymes that contain more than one catalytic domain (CD), as the interactions between CDs, CBMs, and CDs and CBMs can be complex. Our research demonstrates that homogeneous CBMs can have distinct functions in a multimodular enzyme. The tandem CBMs coordinate the CDs in catalytic conflict through their differences in binding affinity, ligand preference, and arrangement within the full-length enzyme. Additionally, although the synergism between mannanase and esterase is widely acknowledged, our study highlights the benefits of integrating the two enzymes into a single entity for the degradation of complex substrates. In summary, these findings enhance our understanding of the intra-synergism of a multimodular enzyme and emphasize the significance of multiple CBMs in this context.

Structure and enzymatic characterization of CelD endoglucanase from the anaerobic fungus Piromyces finnis

Anaerobic fungi found in the guts of large herbivores are prolific biomass degraders whose genomes harbor a wealth of carbohydrate-active enzymes (CAZymes), of which only a handful are structurally or biochemically characterized. Here, we report the structure and kinetic rate parameters for a glycoside hydrolase (GH) family 5 subfamily 4 enzyme (CelD) from Piromyces finnis, a modular, cellulosome-incorporated endoglucanase that possesses three GH5 domains followed by two C-terminal fungal dockerin domains (double dockerin). We present the crystal structures of an apo wild-type CelD GH5 catalytic domain and its inactive E154A mutant in complex with cellotriose at 2.5 and 1.8 Å resolution, respectively, finding the CelD GH5 catalytic domain adopts the (β/α)8-barrel fold common to many GH5 enzymes. Structural superimposition of the apo wild-type structure with the E154A mutant-cellotriose complex supports a catalytic mechanism in which the E154 carboxylate side chain acts as an acid/base and E278 acts as a complementary nucleophile. Further analysis of the cellotriose binding pocket highlights a binding groove lined with conserved aromatic amino acids that when docked with larger cellulose oligomers is capable of binding seven glucose units and accommodating branched glucan substrates. Activity analyses confirm P. finnis CelD can hydrolyze mixed linkage glucan and xyloglucan, as well as carboxymethylcellulose (CMC). Measured kinetic parameters show the P. finnis CelD GH5 catalytic domain has CMC endoglucanase activity comparable to other fungal endoglucanases with kcat = 6.0 ± 0.6 s-1 and Km = 7.6 ± 2.1 g/L CMC. Enzyme kinetics were unperturbed by the addition or removal of the native C-terminal dockerin domains as well as the addition of a non-native N-terminal dockerin, suggesting strict modularity among the domains of CelD. KEY POINTS: • Anaerobic fungi host a wealth of industrially useful enzymes but are understudied. • P. finnis CelD has endoglucanase activity and structure common to GH5_4 enzymes. • CelD's kinetics do not change with domain fusion, exhibiting high modularity.

Biochemical and Regulatory Analyses of Xylanolytic Regulons in Caldicellulosiruptor bescii Reveal Genus-Wide Features of Hemicellulose Utilization

Caldicellulosiruptor species scavenge carbohydrates from runoff containing plant biomass that enters hot springs and from grasses that grow in more moderate parts of thermal features. While only a few Caldicellulosiruptor species can degrade cellulose, all known species are hemicellulolytic. The most well-characterized species, Caldicellulosiruptor bescii, decentralizes its hemicellulase inventory across five different genomic loci and two isolated genes. Transcriptomic analyses, comparative genomics, and enzymatic characterization were utilized to assign functional roles and determine the relative importance of its six putative endoxylanases (five glycoside hydrolase family 10 [GH10] enzymes and one GH11 enzyme) and two putative exoxylanases (one GH39 and one GH3) in C. bescii. Two genus-wide conserved xylanases, C. bescii XynA (GH10) and C. bescii Xyl3A (GH3), had the highest levels of sugar release on oat spelt xylan, were in the top 10% of all genes transcribed by C. bescii, and were highly induced on xylan compared to cellulose. This indicates that a minimal set of enzymes are used to drive xylan degradation in the genus Caldicellulosiruptor, complemented by hemicellulolytic inventories that are tuned to specific forms of hemicellulose in available plant biomasses. To this point, synergism studies revealed that the pairing of specific GH family proteins (GH3, -11, and -39) with C. bescii GH10 proteins released more sugar in vitro than mixtures containing five different GH10 proteins. Overall, this work demonstrates the essential requirements for Caldicellulosiruptor to degrade various forms of xylan and the differences in species genomic inventories that are tuned for survival in unique biotopes with variable lignocellulosic substrates. IMPORTANCE Microbial deconstruction of lignocellulose for the production of biofuels and chemicals requires the hydrolysis of heterogeneous hemicelluloses to access the microcrystalline cellulose portion. This work extends previous in vivo and in vitro efforts to characterize hemicellulose utilization by integrating genomic reconstruction, transcriptomic data, operon structures, and biochemical characteristics of key enzymes to understand the deployment and functionality of hemicellulases by the extreme thermophile Caldicellulosiruptor bescii. Furthermore, comparative genomics of the genus revealed both conserved and divergent mechanisms for hemicellulose utilization across the 15 sequenced species, thereby paving the way to connecting functional enzyme characterization with metabolic engineering efforts to enhance lignocellulose conversion.

Biosynthesis of value-added bioproducts from hemicellulose of biomass through microbial metabolic engineering

Hemicellulose is the second most abundant carbohydrate in lignocellulosic biomass and has extensive applications. In conventional biomass refinery, hemicellulose is easily converted to unwanted by-products in pretreatment and therefore can't be fully utilized. The present study aims to summarize the most recent development of lignocellulosic polysaccharide degradation and fully convert it to value-added bioproducts through microbial and enzymatic catalysis. Firstly, bioprocess and microbial metabolic engineering for enhanced utilization of lignocellulosic carbohydrates were discussed. The bioprocess for degradation and conversion of natural lignocellulose to monosaccharides and organic acids using anaerobic thermophilic bacteria and thermostable glycoside hydrolases were summarized. Xylose transmembrane transporting systems in natural microorganisms and the latest strategies for promoting the transporting capacity by metabolic engineering were summarized. The carbon catabolite repression effect restricting xylose utilization in microorganisms, and metabolic engineering strategies developed for co-utilization of glucose and xylose were discussed. Secondly, the metabolic pathways of xylose catabolism in microorganisms were comparatively analyzed. Microbial metabolic engineering for converting xylose to value-added bioproducts based on redox pathways, non-redox pathways, pentose phosphate pathway, and improving inhibitors resistance were summarized. Thirdly, strategies for degrading lignocellulosic polysaccharides and fully converting hemicellulose to value-added bioproducts through microbial metabolic engineering were proposed.

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Hemicellulose is the main carbohydrate of biomass and has valuable applications.

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Hemicellulose is underutilized in conventional biomass refinery and pretreatment.

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Microbial and enzymatic catalysis were applied for hemicellulose utilization.

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Xylose is converted to value-added bioproducts by metabolic engineering.

Contributions and characteristics of two bifunctional GH43 β-xylosidase /α-L-arabinofuranosidases with different structures on the xylan degradation of Paenibacillus physcomitrellae strain XB

Xylan is one of the major polymeric hemicellulosic constituents of lignocellulosic biomass, and its effective utilization by microorganisms is crucial for the economical production of biofuels. In this study, Paenibacillus physcomitrellae XB exhibited different xylan degradation ability on different substrates of corncob xylan (CCX), oat spelt xylan (OSX), wheat flour arabinoxylan (AX) and beech wood xylan (BWX). The RT-QPCR result showed that two genes (Pph_0602 and Pph_2344) belonging to the glycoside hydrolase family 43 were up-regulated more than 5-fold on CCX and xylose. Substrate-specific assays with purified proteins Ppxyl43A (Pph_0602) and Ppxyl43B (Pph_2344) revealed that both exhibited β-xylosidase activity toward the chromogenic substrate p-nitrophenyl-β-D-xylopyranoside, and α-L-arabinofuranosidase activity toward p-nitrophenyl-α-L-arabinofuranoside, indicating their bifunctionality. By testing their degradation characteristics on different natural substrates, it was found that both Ppxyl43A and Ppxyl43B showed similar degradation ability on CCX and OSX. Both enzymes could hydrolyze xylohexaose and xylobiose completely to xylose, but could not hydrolyze BWX and AX, suggesting they mainly hydrolyze xylo-oligosaccharides by β-xylosidase activity. Further analysis showed that both of them displayed very high pH stability and thermostability on the β-xylosidase activity, but Ppxy143B exhibited wider pH and temperature ranges, higher pH and temperature stability, was less influenced by metal ions, and had a slower start-up response than Ppxyl43A. Given their predicted structure, it is likely that the enzymatic differences between Ppxyl43A and Ppxyl43B might be related to the extra C-terminus domain (GH43_C2) in Ppxyl43B, which could enhance the enzymatic stability while restricting the substrates' or metal ions' access to the active sites of Ppxyl43B. In conclusion, both Ppxyl43A and Ppxyl43B were β-xylosidase/α-L-arabinofuranosidase bifunctional enzymes and might be useful in xylan biomass conversion, especially in the hydrolysis of xylo-oligosaccharides into xylose.

Extremophilic Prokaryotic Endoxylanases: Diversity, Applicability, and Molecular Insights

Extremophilic endoxylanases grabbed attention in recent years due to their applicability under harsh conditions of several industrial processes. Thermophilic, alkaliphilic, and acidophilic endoxylanases found their employability in bio-bleaching of paper pulp, bioconversion of lignocellulosic biomass into xylooligosaccharides, bioethanol production, and improving the nutritious value of bread and other bakery products. Xylanases obtained from extremophilic bacteria and archaea are considered better than fungal sources for several reasons. For example, enzymatic activity under broad pH and temperature range, low molecular weight, cellulase-free activity, and longer stability under extreme conditions of prokaryotic derived xylanases make them a good choice. In addition, a short life span, easy cultivation/harvesting methods, higher yield, and rapid DNA manipulations of bacterial and archaeal cells further reduces the overall cost of the product. This review focuses on the diversity of prokaryotic endoxylanases, their characteristics, and their functional attributes. Besides, the molecular mechanisms of their extreme behavior have also been presented here.

Endo-xylanases from Cohnella sp. AR92 aimed at xylan and arabinoxylan conversion into value-added products

The genus Cohnella belongs to a group of Gram-positive endospore-forming bacteria within the Paenibacillaceae family. Although most species were described as xylanolytic bacteria, the literature still lacks some key information regarding their repertoire of xylan-degrading enzymes. The whole genome sequence of an isolated xylan-degrading bacterium Cohnella sp. strain AR92 was found to contain five genes encoding putative endo-1,4-β-xylanases, of which four were cloned, expressed, and characterized to better understand the contribution of the individual endo-xylanases to the overall xylanolytic properties of strain AR92. Three of the enzymes, CoXyn10A, CoXyn10C, and CoXyn11A, were shown to be effective at hydrolyzing xylans-derived from agro-industrial, producing oligosaccharides with substrate conversion values of 32.5%, 24.7%, and 10.6%, respectively, using sugarcane bagasse glucuronoarabinoxylan and of 29.9%, 19.1%, and 8.0%, respectively, using wheat bran-derived arabinoxylan. The main reaction products from GH10 enzymes were xylobiose and xylotriose, whereas CoXyn11A produced mostly xylooligosaccharides (XOS) with 2 to 5 units of xylose, often substituted, resulting in potentially prebiotic arabinoxylooligosaccharides (AXOS). The endo-xylanases assay displayed operational features (temperature optima from 49.9 to 50.4 °C and pH optima from 6.01 to 6.31) fitting simultaneous xylan utilization. Homology modeling confirmed the typical folds of the GH10 and GH11 enzymes, substrate docking studies allowed the prediction of subsites (- 2 to + 1 in GH10 and - 3 to + 1 in GH11) and identification of residues involved in ligand interactions, supporting the experimental data. Overall, the Cohnella sp. AR92 endo-xylanases presented significant potential for enzymatic conversion of agro-industrial by-products into high-value products.Key points• Cohnella sp. AR92 genome encoded five potential endo-xylanases.• Cohnella sp. AR92 enzymes produced xylooligosaccharides from xylan, with high yields.• GH10 enzymes from Cohnella sp. AR92 are responsible for the production of X2 and X3 oligosaccharides.• GH11 from Cohnella sp. AR92 contributes to the overall xylan degradation by producing substituted oligosaccharides.

Autodisplay of an endo-1,4-β-xylanase from Clostridium cellulovorans in Escherichia coli for xylans degradation

The goal of this work was the autodisplay of the endo β-1,4-xylanase (XynA) from Clostridium cellulovorans in Escherichia coli using the AIDA system to carry out whole-cell biocatalysis and hydrolysate xylans. For this, pAIDA-xynA vector containing a synthetic xynA gene was fused to the signal peptide of the toxin subunit B Vibro cholere (ctxB) and the auto-transporter of the synthetic aida gene, which encodes for the connector peptide and β-barrel of the auto-transporter (AT-AIDA). E. coli TOP10 cells were transformed and the biocatalyst was characterized using beechwood xylans as substrate. Optimal operational conditions were temperature of 55 °C and pH 6.5, and the Michaelis-Menten catalytic constants V_max and K_m were 149 U/g_DCW and 6.01 mg/mL, respectively. Xylanase activity was inhibited by Cu²⁺, Zn²⁺ and Hg²⁺ as well as EDTA, detergents, and organic acids, and improved by Ca²⁺, Co²⁺ and Mn²⁺ ions. Ca²⁺ ion strongly enhanced the xylanolytic activity up to 2.4-fold when 5 mM CaCl₂ were added. Also, Ca²⁺ improved enzyme stability at 60 and 70 °C. Results suggest that pAIDA-xynA vector has the ability to express functional xylanase to perform whole-cell biocatalysis in order to hydrolysate xylans from hemicellulose feedstock.

Characterization of a novel GH10 xylanase with a carbohydrate binding module from Aspergillus sulphureus and its synergistic hydrolysis activity with cellulase

A study was carried out to investigate the characterization of a novel Aspergillus sulphureus JCM01963 xylanase (AS-xyn10A) with a carbohydrate binding module (CBM) and its application in degrading alkali pretreated corncob, rapeseed meal and corn stover alone and in combination with a commercial cellulase. In this study, the 3D structure of AS-xyn10A, which contained a CBM at C-terminal. AS-xyn10A and its CBM-truncated variant (AS-xyn10A-dC) was codon-optimized and over-expressed in Komagaella phaffii X-33 (syn. Pichia pastoris) and characterized with optimal condition at 70 °C and pH 5.0, respectively. AS-xyn10A displayed high activity to xylan extracted from corn stover, corncob, and rapeseed meal. The concentration of hydrolyzed xylo-oligosaccharides (XOSs) reached 1592.26 μg/mL, 1149.92 μg/mL, and 621.86 μg/mL, respectively. Xylobiose was the main product (~70%) in the hydrolysis mixture. AS-xyn10A significantly synergized with cellulase to improve the hydrolysis efficiency of corn stover, corncob, and rapeseed meal to glucose. The degree of synergy (DS) was 1.32, 1.31, and 1.30, respectively. Simultaneously, XOSs hydrolyzed with AS-xyn10A and cellulase was improved by 46.48%, 66.13% and 141.45%, respectively. In addition, CBM variant decreased the yields of xylo-oligosaccharide and glucose in rapeseed meal degradation. This study provided a novel GH10 endo-xylanase, which has potential applications in hydrolysis of biomass.

Characterization of efficient xylanases from industrial-scale pulp and paper wastewater treatment microbiota

Xylanases are widely used enzymes in the food, textile, and paper industries. Most efficient xylanases have been identified from lignocellulose-degrading microbiota, such as the microbiota of the cow rumen and the termite hindgut. Xylanase genes from efficient pulp and paper wastewater treatment (PPWT) microbiota have been previously recovered by metagenomics, assigning most of the xylanase genes to the GH10 family. In this study, a total of 40 GH10 family xylanase genes derived from a certain PPWT microbiota were cloned and expressed in Escherichia coli BL21 (DE3). Among these xylanase genes, 14 showed xylanase activity on beechwood substrate. Two of these, PW-xyl9 and PW-xyl37, showed high activities, and were purified to evaluate their xylanase properties. Values of optimal pH and temperature for PW-xyl9 were pH 7 and 60 ℃, respectively, while those for PW-xyl37 were pH 7 and 55 ℃, respectively; their specific xylanase activities under optimal conditions were 470.1 U/mg protein and 113.7 U/mg protein, respectively. Furthermore, the Km values of PW-xyl9 and PW-xyl37 were determined as 8.02 and 18.8 g/L, respectively. The characterization of these two xylanases paves the way for potential application in future pulp and paper production and other industries, indicating that PPWT microbiota has been an undiscovered reservoir of efficient lignocellulase genes. This study demonstrates that a metagenomic approach has the potential to screen efficient xylanases of uncultured microorganisms from lignocellulose-degrading microbiota. In a similar way, other efficient lignocellulase genes might be identified from PPWT treatment microbiota in the future.

Enzymatic characterization of a novel thermostable and alkaline tolerant GH10 xylanase and activity improvement by multiple rational mutagenesis strategies

Thermo-alkaline xylanases are widely applied in paper pulping industry. In this study, a novel thermostable and alkaline tolerant GH10 xylanase (Xyn30Y5) gene from alkaliphilic Bacillus sp. 30Y5 was cloned and the surface-layer homology (SLH) domains truncated enzyme (Xyn30Y5-SLH) was expressed in Escherichia coli. The purified Xyn30Y5-SLH was most active at 70 °C and pH 7.0 and showed the highest specific activity of 349.4 U mg^-1. It retained more than 90% activity between pH 6.0 to 9.5 and was stable at pH 6.0-10.0. To improve the activity, 47 mutants were designed based on eight rational strategies and 21 mutants showed higher activity. By combinatorial mutagenesis, the best mutant 3B demonstrated specific activity of 1016.8 U mg^-1 with a doubled catalytic efficiency (k_cat/K_m) and RA60_1/2h value, accompanied by optimal pH shift to 8.0. The molecular dynamics simulation analysis indicated that the increase of flexibility of α5 helix and loop7 located near to the catalytic residues is likely responsible for its activity improvement. And the decrease of flexibility of the most unstable regions is vital for the thermostablity improvement. This work provided not only a novel thermostable and alkaline tolerant xylanase with industrial application potential but also an effective mutagenesis strategy for xylanase activity improvement.

In silico screening and experimental analysis of family GH11 xylanases for applications under conditions of alkaline pH and high temperature

BACKGROUND

Xylanases are one of the most extensively used enzymes for biomass digestion. However, in many instances, their use is limited by poor performance under the conditions of pH and temperature required by the industry. Therefore, the search for xylanases able to function efficiently at alkaline pH and high temperature is an important objective for different processes that use lignocellulosic substrates, such as the production of paper pulp and biofuels.

RESULTS

A comprehensive in silico analysis of family GH11 sequences from the CAZY database allowed their phylogenetic classification in a radial cladogram in which sequences of known or presumptive thermophilic and alkalophilic xylanases appeared in three clusters. Eight sequences from these clusters were selected for experimental analysis. The coding DNA was synthesized, cloned and the enzymes were produced in E. coli. Some of these showed high xylanolytic activity at pH values > 8.0 and temperature > 80 °C. The best enzymes corresponding to sequences from Dictyoglomus thermophilum (Xyn5) and Thermobifida fusca (Xyn8). The addition of a carbohydrate-binding module (CBM9) to Xyn5 increased 4 times its activity at 90 °C and pH > 9.0. The combination of Xyn5 and Xyn8 was proved to be efficient for the saccharification of alkali pretreated rice straw, yielding xylose and xylooligosaccharides.

CONCLUSIONS

This study provides a fruitful approach for the selection of enzymes with suitable properties from the information contained in extensive databases. We have characterized two xylanases able to hydrolyze xylan with high efficiency at pH > 8.0 and temperature > 80 °C.

Exploration of Two Pectate Lyases from Caldicellulosiruptor bescii Reveals that the CBM66 Module Has a Crucial Role in Pectic Biomass Degradation

Pectin deconstruction is the initial step in breaking the recalcitrance of plant biomass by using selected microorganisms that encode pectinolytic enzymes. Pectate lyases that cleave the α-1,4-galacturonosidic linkage of pectin are widely used in industries such as papermaking and fruit softening. However, there are few reports on pectate lyases with good thermostability. Here, two pectate lyases (CbPL3 and CbPL9) from a hyperthermophilic bacterium, Caldicellulosiruptor bescii, belonging to family 3 and family 9 polysaccharide lyases, respectively, were investigated. The biochemical properties of the two CbPLs were shown to be similar under optimized conditions of 80°C to 85°C and pH 8 to 9. However, the degradation products from pectin and polygalacturonic acids (pGAs) were different. A family 66 carbohydrate-binding module (CbCBM66) located in the N terminus of the two CbPLs shares 100% amino acid identity. A CbCBM66-truncated mutant of CbPL9 showed lower activities than the wild type, whereas CbPL3 with a CbCBM66 knockout portion was reported to have enhanced activities, thereby revealing the different effect of CbCBM66. Prediction by the I-TASSER server revealed that CbCBM66 is structurally close to BsCBM66 from Bacillus subtilis; however, the COFACTOR and COACH programs indicated that the substrate-binding sites between CbCBM66 and BsCBM66 are different. Furthermore, a substrate-binding assay indicated that the catalytic domains in the two CbPLs had strong affinities for pectate-related substrates, but CbCBM66 showed a weak interaction with a number of lignocellulosic carbohydrates. Finally, scanning electron microscopy (SEM) analysis and a total reducing sugar assay showed that the two enzymes could improve the saccharification of switchgrass. The two CbPLs are impressive sources for the degradation of plant biomass.IMPORTANCE Thermophilic proteins could be implemented in diverse industrial applications. We sought to characterize two pectate lyases, CbPL3 and CbPL9, from a thermophilic bacterium, Caldicellulosiruptor bescii The two enzymes share a high optimum temperature, a low optimum pH, and good thermostability at the evaluated temperature. A family 66 carbohydrate-binding module (CbCBM66) was identified in the two CbPLs, sharing 100% amino acid identity. The deletion of CbCBM66 dramatically decreased the activity of CbPL9 but increased the activity and thermostability of CbPL3, suggesting different roles of CbCBM66 in the two enzymes. Moreover, the degradation products of the two CbPLs were different. These results revealed that these enzymes could represent potential pectate lyases for applications in the paper and textile industries.

Insights into Thermophilic Plant Biomass Hydrolysis from Caldicellulosiruptor Systems Biology

Plant polysaccharides continue to serve as a promising feedstock for bioproduct fermentation. However, the recalcitrant nature of plant biomass requires certain key enzymes, including cellobiohydrolases, for efficient solubilization of polysaccharides. Thermostable carbohydrate-active enzymes are sought for their stability and tolerance to other process parameters. Plant biomass degrading microbes found in biotopes like geothermally heated water sources, compost piles, and thermophilic digesters are a common source of thermostable enzymes. While traditional thermophilic enzyme discovery first focused on microbe isolation followed by functional characterization, metagenomic sequences are negating the initial need for species isolation. Here, we summarize the current state of knowledge about the extremely thermophilic genus Caldicellulosiruptor, including genomic and metagenomic analyses in addition to recent breakthroughs in enzymology and genetic manipulation of the genus. Ten years after completing the first Caldicellulosiruptor genome sequence, the tools required for systems biology of this non-model environmental microorganism are in place.